FIG. 1 is a cross-sectional view of an aluminum electrolytic capacitor. As shown in FIG. 1, the anode foil 1 and cathode foil 2 are wound together with the separator 4 made of Manila hemp or Kraft paper to form a capacitor element. This capacitor element is placed together with a driving electrolytic solution (hereinafter, electrolytic solution) in a bottomed metal case 5 and an opening of the foregoing metal case is closed by sealing with a sealing material 6.
The anode foil 2 is produced by anodizing and forming a dielectric layer on an aluminum foil after expanding (roughening) its surface area by etching.
The electrolytic solution increases the electrostatic capacity through absorption to the anode foil 2. The electrolytic solution helps maintain a low level of leakage current since it can repair the dielectric layer of the anodized aluminum film using its own anodizing ability. Among the properties of the electrolytic solution, electrical conductivity has a significant influence on the impedance of the electrolytic capacitor.
Considering the above mentioned points, highly conductive electrolytic solutions stable at high temperatures may be used especially for a low impedance capacitor operating in an environmental temperature of not less than 105.degree. C. and having a rated voltage of not more than 100V in particular. Such electrolytic solutions typically use .gamma.-butyrolactone which has good low temperature and anodizing properties as a solvent, and quaternary ammonium salts of phthalic acid and maleic acid as electrolytes (refer to Japanese Laid Open Patent Publication No. S62-145713 and Japanese Laid Open Patent Publication No. S62-145715).
However, if the electrolytic capacitor using .gamma.-butyrolactone as a solvent and quaternary ammonium salts of phthalic acid and maleic acid as an electrolyte, is continuously used in a high-humidity atmosphere, a strong alkali compound is generated at the cathode. The strong alkali compound corrodes the cathode lead and the sealing material 6 which contacts the cathode lead. The corrosion causes leakage of the electrolytic solution from the capacitor.
The effective alternative to avoid the foregoing problem is to use an electrolytic solution which does not generate large amounts of alkali compounds, namely the electrolytic solution using ethylene glycol and water as solvent and ammonia salts such as ammonium adipate as electrolyte.
The electrolytic solution using .gamma.-butyrolactone as a solvent has a flash point of approximately 100.degree. C. Therefore, it is impossible to avoid the possibility of ignition occurring when the electrolytic solution is released due to malfunction of the electric device or similar reasons when used in an environment above 100.degree. C.
With regard to low rating voltage electrolytic capacitors of 100V or less at an operating temperature of 85.degree. C., an electrolytic solution in which the solvent is a mixture of ethylene glycol and water and electrolyte, ammonium salts such as ammonium adipate and so on, can be used. Water is used to enhance electric conductivity. However, this kind of capacitor has the problem of not being able to be used for long periods of time at and above the boiling point of one of the solvents or water (100.degree. C.). In experiments, a large amount of hydrogen gas was generated in a rated voltage test at 110.degree. C. due to the reaction between aluminum and water. Generated hydrogen raised the internal pressure of the capacitor. As a result, in some cases, the safety valve at the bottom of the aluminum case broke. In a no-load shelf test at 110.degree. C., the rate of change of post-test leakage current within 1000 hours was found to exceed +5000%.
In order to solve these problems a variety of methods have been proposed. Among them is a method in which various phosphor compounds are added to the electrolytic solution to suppress the reaction between the electrode foils and the water. Another method suggests that various nitro-compounds be added to absorb generated hydrogen gas. However, even with these methods, when dealing with electrolytic capacitors of rated voltage being 100V or less, it is difficult to maintain electric performance for a long time at 100.degree. C. or higher if an electrolytic solution with a water content of over 20% is used.
Moreover, when highly conductive electrolytic solution with water content over 20% is used, chlorine contained in the sealing rubber causes a problem, which is not a problem with an electrolytic solution having water content of less than 20%. Namely, in a long-term load test at high temperatures, the anode aluminum lead was corroded, sometimes leading to an increase in current leakage or breaking of anode aluminum lead due to corrosion.
The present invention aims to provide highly reliable electrolytic capacitors by solving these problems associating with conventional electrolytic capacitors.